FIG. 13 is a diagram showing the structure of a conventional PWM inverter. In FIG. 13, 1 denotes a controller, 2 denotes a noise reducing PWM generating circuit, 4 denotes a current detecting circuit, 5 denotes a DC power supply, 11 denotes a smoothing capacitor, 101 to 106 denote a switching unit, and 201 to 206 denote a free wheel diode.
With the structure described above, a command and a detection value of an output current from the current detecting circuit 4 are input to the controller 1. The controller 1 creates a PWM switching pattern specialized to reduce a noise by the noise reducing PWM generating circuit 2 in response to their inputs and outputs a command for turning ON/OFF a switching unit.
Description will be given to the PWM switching pattern specialized to reduce a noise. A PWM pulse having a 3-phase and 2-level which is output from the circuit shown in FIG. 13 is illustrated in a spatial vector diagram of FIG. 2, and a general PWM pulse pattern is shown in FIG. 3. Each of symbols a, b, Op and On vectors on the spatial vector diagram of FIG. 2 corresponds to the switching pattern of each phase shown in FIG. 3, and H indicates a state in which an upper switching unit is ON and L indicates a state in which a lower switching unit is ON.
A PWM inverter usually outputs the pattern shown in FIG. 3, and furthermore, outputs a noise reducing pattern shown in FIG. 4 which is obtained by recombining a pulse pattern by the noise reducing PWM generating circuit 2. The noise reducing pattern is equal to the ordinary pattern of FIG. 3 on a time average, and the pulse pattern shown in FIG. 4 in which a pulse is divided, converged and distributed is generated to disperse a noise, whereby a noise reduction can be thus attained.
In the conventional art, however, a normal PWM method uniformly carries out switching for all phases. For this reason, a frequency generating a noise is converged to make an unpleasant noise. If the pulse pattern is complicated based on a noise reducing countermeasure, however, the number of switching operations is considerably increased so that the switching loss of a switching unit is raised and the generation of heat is thus increased. This is a big problem in that an inverter is to be operated safely, and there is a problem in that the lifetime of the switching unit is also influenced.
Therefore, it is an object of the invention to provide a PWM inverter control method capable of freely setting the switching of each of phases, in which the peaks of their noises are dispersed, and furthermore, the number of switching operations is provided with a lower limit value and an upper limit value to reduce a switching loss to be equal to or smaller than a set value so that a noise reducing countermeasure and energy saving can be consistent with each other.